Electro spinning of submicron diameter polymer filaments

ABSTRACT

An electro spinning process yields uniform, nanometer diameter polymer filaments. A thread-forming polymer is extruded through an anodically biased die orifice and drawn through an anodically biased electrostatic field. A continuous polymer filament is collected on a grounded collector. The polymer filament is linearly oriented and highly uniform in quality. The filament is particularly useful for weaving body armor, for chemical/biological protective clothing, as a biomedical tissue growth support, for fabricating micro sieves and for microelectronics fabrication.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for Governmental purposeswithout the payment of any royalties thereon or therefore.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for spinning a polymer filament. Moreparticularly, the invention relates to a process for forming a polymerfilament in an electrostatic charged field provided by plural, spacedelectrodes. The invention also relates to a polymer forming apparatuscomprising a spinneret.

2. Discussion of the Related Art

The invention relates to a process for the production of polymerfilaments, fibers and other very fine polymer extrudates from athread-forming polymer. A solid polymer is converted to a fluid state sothat it will pass under pressure through a fine extrusion die orifice. Acontinuous liquid phase filament is extruded and drawn though a zone inwhich solvent evaporation and cooling takes place, causing the filamentto solidify and form a continuous, solid filament. The solid filament iscollected by means such as a rotating drum, moving belt, water bath, andthe like or a combination thereof. In a filament extrusion apparatus,the die orifice is oriented to pass the liquid phase filament directlytoward the collection means. This facilitates a linear orientation inthe solidifying polymer filament. Drawing and annealing also facilitatethe linear orientation of the fiber.

In the electro spinning process, solvent evaporation from the filament,filament cooling or both take place in the zone between the extrusiondie and the filament collection means. This zone is biased to maintainan electrostatic field.

The extrusion die and the filament collection means are eachelectrically conductive. An electric potential difference is maintainedbetween them. In commercial practice the bias is in the range of 5,000to 15,000 volts, often 5,000 to 10,000 volts. An electro spinningapparatus usually has a positively biased die and a grounded collectionmeans. A positively biased die with a negatively biased collection meanshas also been used. The polarity necessitates a minimum separationbetween the die and collection means to prevent arcing across the zone.In this sense, the electrostatic field is elongated or longitudinalbetween the two conductors. Reference to the elongated or longitudinalelectrostatic field is also consistent with the longitudinal orientationof the filament as it is drawn linearly in the electrostatic field.

The polymer filament is drawn from the die orifice to the collectionmeans under the influence of the electrostatic field. As a result, theextruded molten filament is subjected along its length to anelectrostatic field. The strength of the electrostatic field decreasesexponentially with the distance between the electrodes. Accordingly, thedrawing rate from the anodic die to the cathodic drawing means variesalong the filament length due to the variation in the electrostaticcharge on the filament.

U.S. Pat. Nos. 1,975,504; 2,158,416; and 2,323,025 to A. Formhalsdisclose a process and apparatus for preparing artificial threads. Thesepioneer patents disclose the essential elements of the process,including a pair of spaced electrodes. These electrodes produce anelectrostatic field through which an extruded filament is drawn. Meansis provided for varying the rate of filament collection. The filament isspun into yarn by mechanical spinning means.

SUMMARY OF THE INVENTION

In accordance with the invention, a process is provided for electrospinning a polymer filament, fiber or the like in an electrostatic fieldcreated between a biased polymer extrusion die and a collection means.

A thread-forming polymer is liquefied and extruded through a dieorifice. The extruded filament is drawn while solidifying, through theelectrostatic field. A solid filament is collected on the collectionmeans.

The electrostatic field is sequentially biased along the length of theliquid filament. The polarity of the electrostatic field is the same asthat of the die. The extruded filament is exposed to the sequential biasas it solidifies. As a result, the liquid state filament and theresulting solid filament product have a uniform linear molecularorientation longitudinally along the filament. Therefore the solidfilament product tends to have a uniform linear molecular orientation.

Solid filaments produced according to the process has physicalproperties which make them particularly useful for making protectiveclothing such as chemical and biological protective clothing, lightweight personal body armor and the like. The solid filament also hasutility in applications in uses such as tissue growth medium,particulate filters and for optical and electronic applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an electro spinning spinneret andelectrode assembly.

FIG. 2 is a graphical representation of electrostatic field strengthwith respect to distance from the die orifice to the collecting means.

DETAILED DESCRIPTION OF THE INVENTION

The polymers useful in the electro spinning process include any of thethread-forming polymers known in the art for this utility. These includeespecially polyamides, polyesters, polyolefins, and polyacrylonitrile.Suitable polyamides are nylon 6, nylon 6/6, nylon 6/9, nylon 6/10, nylon6/12, nylon 11, nylon 12, copolymers thereof and mixtures thereof.Suitable polyesters are polyalkylene terephthalate and polyalkylenenaphthalates, particularly polyethylene terephthalate. Suitablepolyolefins are polymers of C2 to C10 olefins, in particularpolyethylene, polyoxyethylene, polypropylene and polyoxypropylene andcopolymers thereof and mixtures thereof with polymethylmethacrylate.

Solvents useful in the electro spinning process include any of thosesolvents which are which are effective for use in liquefyingthread-forming polymers. The solvent must be stable in the liquid statein mixtures with the polymer at electro spinning process operatingtemperatures. The solvent must also rapidly evaporate from the surfaceof the polymer while the polymer is drawn to a thin filament or fiber.Such solvents include acetone, acetic ether, benzene, methyl alcohol,ethyl alcohol, propyl alcohol and the like and mixtures thereof. Wateris a particularly useful solvent in the process. Typically, polymer isdissolved in a solvent or combination of solvents in an amount such thatthe solution passing through the die orifice contains about 1% to 20% byweight thread-forming polymer in solvent and has a viscosity of about 1to 200 Poise. Electro spinning of a feedstock fluid comprising polymerdissolved in solvent is referred to as electro spinning from solution.

Although less common, melted polymers which display a Melt Index ofabout 0.5 to 2.0 in the absence of solvent are usable in the processwithout solvent. Melt Index is measured according to ASTM D-1238, TestMethods for Flow Rates of Thermoplastics by Extrusion Plastometer. Suchpolymers are heated in the mixer to a temperature, typically in therange of 180° to 350° Centigrade, at which they melt and display therequired Melt Index of about 0.5 to 2.0. Electro spinning of a feedstockcomprising melted polymer in the absence of solvent is referred to aselectro spinning from a molten polymer or polymer melt.

Both electro spinning from solution and electro spinning from moltenpolymer produce a continuous solid polymer filament of 5 to 1000nanometers in diameter, preferably 100 to 500 nanometers in diameter.

It is understood that additives such as dyes, pigments, lubricants,nucleating agents, antioxidants, ultraviolet light stabilizers,antistatic agents, soil resistance agents, stain resistance agents antimicrobial agents, flame retardants, conductive particles and the likeare added to polymer and solvent in the mixer to form a homogenousliquid polymer mixture. The concentration of these additives is chosenaccording to the desired properties of the final filament, fiber or thelike.

Commercial mixers are available for liquefying thread-forming polymer bymixing polymer and solvent together, optionally with controlled heating.Electric stirrers are commonly used for batch mixing. Extruders are usedin commercial and laboratory practice for melting and mixing polymer,and any additives and for extruding the thread-forming polymer into afilament, thread or the like.

Inventors adopt the convention herein that the noun “bias” refers to anapplied voltage. The verb “bias” means to apply a negative or positivevoltage to a body. The word “charge” refers to a definite quantity ofelectricity, particularly excess or a deficiency of electrons on a body.Furthermore, a bias is applied to the electrodes. This produces anelectric field. The electric field induces a charge in the polymersolution. The electric field exerts a force on the induced charge in thepolymer solution that causes a conical meniscus to form and to be drawntoward the collecting means.

Attention is drawn to FIG. 1, a schematic diagram of the electrospinning process of the invention. FIG. 1 discloses the essentialelements of the inventive method and apparatus for carrying out theinvention.

A solid polymer in bead form or reduced to chip form is passed to mixer10 along with solvent and optional additives. Mechanical mixing may addsufficient heat, or the mechanical heat of mixing may be supplemented bymeans of electrical heating to achieve the desired viscosity. By way ofexample the liquefied polymer may comprise 4% to 13% by weightpolyoxyethylene polymer having an average molecular weight of about400,000 dissolved in water. The liquid polymer 15 is passed to a heateddie 20. Die 20 is shown here by way of example as a single verticallydownward passage capillary having a lower outlet orifice 30 diameter ofabout 5 to 1,000 nanometers, typically 100 to 500 nanometers. Inpractice, the die may comprise a plurality of individual capillaries anddie orifices. For example, a single die may comprise 5 to 10 capillarieseach having a 1-2 millimeter orifice. The die is also provided withmeans for maintaining the liquefied polymer under sufficient pressure toforce the polymer through the die orifices at a rate of about 1milliliter/hour/die orifice, e.g. 0.5 milliliter/hour/die orifice.

Die 20 is integrally attached to, but electrically isolated from mixer10. Die 20 is made of electrically conductive material. In thealternative, an electrode may be positioned in contact with theliquefied polymer. A biased electrode positioned in the liquid polymeris the functional equivalent of biasing die 20. It is typical to biasthe die by biasing means 22 via anode connector 24 to an electrospinning anodic voltage of 5,000 to 15,000 volts. Of course the voltageapplied is variable within the operating range and is selected bymethods well known in the art in order to bring the continuous processto a steady state that produces a continuous polymer filament having thedesired physical properties. The die has a fixed diameter orifice. Itmay be replaced with a die of a different fixed diameter orifice.Voltage to the die is adjusted during start-up to yield a continuousfilament drawn vertically downward along a linearly elongated path.Process start-up is unsteady state by its nature. However, skilledtechnicians can start up a process apparatus and bring it to steadystate production quickly enough to produce only small amounts of scrappolymer filament. Any scrap is recycled to the mixer.

Vertically 3 to 100 centimeters below the orifice 30 is collecting means70. Filament collection means 70 may be any apparatus suitable for thepurpose of collecting a continuous nanometer diameter filament, such asa rotating drum, a conveyer belt, an electrically biased plate, a biasedweb, a water bath, and any combination thereof. Collection means 70 ismade of electrically conducting material and optionally may be grounded,indicated by ground 71. Alternatively, collection means 70 is biased bybiasing means 72 via cathode connector 74. For example, collection meansmay be cathodically biased to 5,000 to 15,000 volts, e.g. equal andopposite of the bias on die 20. As discussed, the specific bias voltageselected is a matter of process start up and product quality control.

Between the die 20 and the collecting means 70 is, according to theinvention, a sequentially biased electrostatic field 50. Theelectrostatic field may be biased to 10,000 to 300,000 volts/meter. Forexample, it may be linearly biased to 50,000 to 250,000 volts/meter.

In practice an apparatus may comprise 5 to 10 spinnerets that are spaced1 to 4 centimeters apart. The filaments are typically spun into yarn.Methods for spinning filaments into yarn are well known in the art. Thismay be accomplished in the process by increasing the electric potentialof the middle one or more rings, e.g. electrode 55 b, to cause adjacentfilaments to repel and displace laterally from one another. Thisresolves into a spiraling or corkscrew motion, the functional equivalentof spinning. Electric potential to the following rings is reducedcausing the spiraling filaments to converge at a common center. Thisspinning and recentering is used to spin the filaments into yarn.

In the alternative, the bias of the middle one or more rings can bereduced to induce a small amount of lateral contact between filaments.This also causes spinning of several filaments into yarn. The apparatusused to accomplish spinning provides for independently chargedelectrodes.

This laterally induced instability can also be used to weave fabric.Weaving is accomplished by means of shaped electrodes. For example, thefinal two electrodes before collecting means can be parallel plates thatdirect the filament and control deposition. It is possible to achievecomplicated weaving by means of multiple flat plate electrodes, e.g.four flat plate electrodes.

A liquid state polymer in the capillary of die 20 is positively biasedby contact with anode connector 24. A meniscus forms at the outlet ofthe capillary, and is immediately attracted by the cathodic bias oncollection means 70 to form a conical meniscus 35 having a hemisphericaltip. The term “critical” voltage refers to the electrical potential atwhich the electrostatic force acting on the liquid surface balances theliquid surface tension. This potential is determined by the physicalproperties of the polymer liquid, which include surface tension,dielectric constant and viscosity. Once the critical voltage isexceeded, a fine stream of liquid erupts from the apex of the cone. Thisstream is maintained by a continuous feed of liquid to the capillaryoutlet at rates less than 1 milliliter/hour/die orifice up to about 10milliliter/hour/die orifice.

This fine stream of liquid is adjusted by means of the anode voltage tobe liquid filament 40. The fine stream of liquid has an initial diameterof 50 to 100 microns on start up. The process yields a continuous solidpolymer filament of 5 to 1,000 nanometers in diameter, preferably 100 to500 nanometers in diameter.

The electrostatic attractive force draws liquid filament 40 to thecathodically biased collecting means 70. As the liquid polymer filament40 is drawn, it is subjected to solvent evaporation and/or cooling tothe ambient temperature, measured by thermocouple 42 at a single pointor at multiple points along the filament path. Adjusting ambientpressure and humidity controls the rate of solvent evaporation.Adjusting ambient temperature controls the rate of cooling. As a resultof solvent evaporation and/or cooling, the liquid filament 40 solidifiesinto a solid polymer filament 60.

The linear distance between die 20 and collecting means 70 is 0.5 to1,000 centimeters or more, e.g. 20 centimeters. Criticality has beenfound in this linear distance. It has been found at linear distances ofabout 3 centimeters and greater, that the path of the fiber deviatesfrom straight. Often the deviation is in a generally circular or spiralpath. In a first case where the liquid filament 40 spirals, it can breakinto discontinuous fibers. This is undesirable if a continuous filamentis required.

In a second case of filament spiraling, a non-linear bias can beimparted to the filament. This case may in fact be desirable for fiberswherein a natural wool-like crimp is sought, such as in the manufactureof thread for weaving cloth for clothing and the like. However, anon-linear bias is undesirable for some other applications, such as finevery fine mesh woven cloth.

The reason for this path variation is under investigation. The nanometerdiameter fibers produced are so fine that they are influence by smalltransient changes in physical and/or electrostatic forces. Drawing andannealing impart a linear orientation to the fiber. In industrialpractice the fiber is collected on a rotating drum that by design has arotational velocity greater than the velocity of the fiber leaving thedie. The rate of drawing is controlled by adjusting the velocitydifferential between the fiber and the rotating drum. Any small changein this velocity differential influences the acceleration of a filamentincrement and the degree of linear orientation of that increment.

It is also known that between the anode and the cathode, the attractiveforce on the molten polymer filament decreases exponentially with thelinear distance between the two. This nonlinear relationship isrepresented as curve 85 on FIG. 2. It is theorized that at a certaindistance, the attractive drawing force on the filament is so reducedthat electrical charges on the lateral surface are no longerinsignificant and the charges begin to hunt an alternate ground, pullingthe filament laterally with them. It is known that the attractiveelectrical force between the biased die and the biased collecting meanscauses the liquid polymer at the tip of the capillary to distort into aconical liquid polymer meniscus having a hemispherical tip. The fineliquid filament is drawn from this tip. It is thought that repulsivecharge forces in the capillary result in instability or vibration in theconical meniscus resulting in an oscillation in the resulting liquidfilament.

Applicants have found that a distance of about 3 centimeters from thedie orifice is the point beyond which chaotic or spiraling motion mayoccur in the filament. All of these observations have lead inventors toconclude that there is insufficient restoring force on the drawnfilament for inherent stability to ensure consistent product quality inthe electro spinning process.

The liquid filament 40 is passed through a longitudinally sequentiallybiased electrostatic zone 50. This sequentially biased electrostaticzone 50 overcomes the deficiency in the electrostatic field created in aconventional electro spinning process between the die and the filamentcollecting means. The polymer filaments are passed though a series ofelectrostatically biased electrodes 55 a, 55 b, and 55 c. Theseelectrostatically biased electrodes are charged by means of biasingmeans 52 with a bias having the same polarity as that of the die. Eachof electrodes is sequentially biased at a lower voltage than thepreceding electrode. This can be accomplished by biasing each with thesame biasing means 52 in series, with electrical resistors in thecircuit between the biased electrodes in order to step the voltage down.For example resistors 56 a, 55 b and 56 c may each be rated 50 ohms at30,000 volts. Accordingly, the voltage in electrode 55 a is lower thanthe voltage in die 20. The voltage in electrode 55 b is lower than thevoltage in electrode 55 a. The voltage in electrode 55 c is lower thanthe voltage in electrode 55 b.

Although three electrodes are shown, it is understood that a pluralityof electrodes and a like number of identical resistors can be assembledto provide a sequentially biased electrostatic zone. For example, if nis the number of electrostatically biased electrodes 55 a, 55 b, 55 c, ncan equal 10, 20, 50 or more. The die and collecting means are eachcounted as electrodes. The total number of electrodes in the electrospinning apparatus is then n+2.

The die is counted as the first electrode to which is applied the firstelectrical charge. The first ring in the electrode means is counted asthe second electrode to which is applied the second electrical charge.The last ring in the electrode means is counted as the (n+1)^(th)electrode to which is applied the (n+1)^(th) charge. The (n+1)^(th) ringwill usually be in electrical communication with ground, indicated byground 57, via resistor 56 c. The collecting means is the (n+2)^(th)electrode to which is applied the (n+2)^(th) charge. In industrialpractice the (n+2)^(th) bias will usually be ground as indicated byground 71.

Electrostatic zone 50 can be biased by means of a number, n, ofelectrodes to provide an essentially linearly biased electrostatic zone50 between die 20 and collecting means 70. The electrodes are usuallyuniformly spaced, i.e. there is an equal distance between electrodes.The zone 50 should sufficiently fill the space between die 20 andcollecting means 70 to that liquid polymer 40 is exposed to asequentially biased zone. Once solidified, the polymer molecules retaintheir linear orientation. The resulting solidified filament retains alinear bias.

Solidifying of the liquid filament takes place primarily due to solventloss, and is also due to cooling, or a combination of the two as thefilament passes from the die orifice 30 through electrostatic field 50.

Attention is drawn to FIG. 2, which graphically depicts the bias fieldfrom die charging means 22 to collecting means charging means 70. Thisincludes electrostatic zone 50. Length from tip to target is plotted onthe ordinate in meters. Field strength is plotted on the abscissa involts/meter. Die 20 is at distance 0 meters. Collecting Means 70 is atdistance 0.2 meters. The bias applied by electrode means provides anessentially linear voltage gradient, represented by line 80. Thegradient in an electro spinning process of the prior art is representedby curve 85.

The charged polymer filament can be deposited directly on collectingmeans. In the alternative, the solid filament can be passed betweenparallel plate electrodes. By adjusting the electric potential dropbetween the plates, it is possible to move the filament laterally. If asecond set of parallel plate electrodes is positioned orthogonal to thefirst set of electrode plates, the fiber can be selectively positionedin two dimensions on a collecting means.

Table of Elements in the Drawing

10 Mixer

15 Molten polymer mixture

20 Die

22 Die biasing means

24 Anode

30 Die orifice

35 Conical meniscus

40 Liquid filament

42 Thermocouple

50 Sequentially biased electrostatic field

52 Electrostatic field biasing means

54 Anode connector

55 a, b, c Electrostatically biased electrodes

56 a, b, c Resistors

57 Electrostatic field biasing means ground

60 Polymer filament

70 Collecting means

71 Collecting means ground

72 Collecting means biasing means

74 Cathode connector

80 Linear voltage gradient line

85 Non-linear voltage gradient curve

90 Voltage gradient curve of Example 1

This invention is shown by way of Example.

EXAMPLE 1

An electro spinning apparatus was assembled according to the invention.Polyoxyethylene and water were mixed to form a homogeneous aqueouspolymer fluid. The die was biased to a positive 7,000 volts. A positivebias of 5,000 volts was applied to the eight ring electrode means. Theresulting electric field gradient is represented by line 90 in FIG. 2.An aluminum foil collection surface was biased at negative 10,000 volts.The liquid polymer was drawn vertically downward through the center ofthe eight ring electrode means over a distance of 8 inches (20.3centimeters). The filament was photographed by means of high-speed laserimaging. A uniform filament was produced having a maximum diameter of300 nanometers measured by electron microscopy.

EXAMPLE 2

Voltage to the eight ring electrode means was reduced to 2,500 volts.The liquid polymer filament became unstable, and developed corkscrewpath of travel between the die and the collector. The filament was againphotographed.

Voltage to the electrode means was then increased to 5,000 volts. Thecorkscrew path instability damped out and a linear filament wasreestablished.

The foregoing discussion discloses and describes embodiments of thepresent invention by way of example. One skilled in the art will readilyrecognize from this discussion and from the accompanying drawings andclaims, that various changes, modifications and variations can be madetherein without departing from the spirit and scope of the invention asdefined in the following claims.

What is claimed is:
 1. An electro spinning process for extruding athread-forming polymer and drawing between an electrically charged die,having a first electrical bias and a first polarity and an electricallycharged collecting means to produce a continuous polymer filament,comprising the steps of: liquefying the thread-forming polymer andextruding through a die orifice to form a liquid-state filament, drawingthe liquid-state filament through a longitudinally sequentially biasedelectrostatic field having the same polarity as the first polarity,thereby imparting a bias gradient to the liquid-state filament,solidifying the liquid-state filament to form a linearly oriented solidpolymer filament, and collecting the solid polymer filament on thecollecting means.
 2. The electro spinning process of claim 1 wherein thedie and electrostatic field are positively biased and the collectingmeans is negatively biased.
 3. The electro spinning process of claim 1wherein the die and charged electrostatic field are positively biasedand the collecting means is grounded.
 4. The electro spinning process ofclaim 1 wherein the longitudinally sequentially biased field gradienthas a length of 3 centimeters or more.
 5. The electro spinning processof claim 1 wherein the longitudinally sequentially biased field gradienthas a length of 3 to 100 centimeters.
 6. The electro spinning process ofclaim 1 wherein the longitudinally sequentially biased field gradient islinearly charged.
 7. The electro spinning process of claim 1 wherein thelongitudinally sequentially biased field gradient is linearly charged to10,000 to 300,000 volts/meter.
 8. The electro spinning process of claim1 wherein the longitudinally sequentially biased field gradient islinearly charged to 50,000 to 250,000 volts/meter.
 9. The method ofclaim 1 wherein the polymer filament is extruded to a diameter of 100 to500 nanometers.